Stainless steel with high resistance against corrosion and welding cracks

ABSTRACT

Stainless steel with high resistance against corrosion and welding cracks, consisting of carbon, silicon, manganese, nickel, chromium, copper and the remainder of iron with an inevitable amount of impurity. The contents of the aforesaid ingredient elements of the stainless steel are so related with each other that the value of the following austenite/ferrite ratio is kept equal to or less than 1.06.   The stainless steel may contain a limited amount of molybdenum, niobium, titanium, and tantalum, with a modified austenite/ferrite ratio. The stainless steel is advantageously used to make heat exchangers in processes employing automatic tungsten inert gas arc welding techniques.

United" States Patent [1 1 Yokota et a1.

1 1 STAINLESS STEEL WITH HIGH RESISTANCE AGAINST CORROSION AND WELDING CRACKS [73] Assignee: Nippon Yakin Kogyo Company Limited, Tokyo, Japan [22] Filed: Oct. 6,1972 [21] Appl. No.1 295,733

Related Us, Application Data [63] Continuation of Ser. No. 22,173, March 24, 1970,

abandoned.

[52] US. CI 29/l96.1, 75/125, 219/137 [51] Int. Cl B32b 15/00, C22c 39/20 [58] Field of Search 29/1961; 219/37; 75/125 [56] References Cited I UNITED STATES PATENTS 2,914,346 11/1959 Ryderflw, 29/1961 2,963,129 12/1960 Eberle 4 29/1961 3,019,513 2/1962 Hornaday 29/1961 3,357,868

12/1967 Tanczyn 75/125 [451 Jan. 15, 1974 3,438,769 4/1967 Kawahata 75/125 3,610,876 10/1971 Bhat 1. 219/137 2,712,317 1/1973 Hayashi 29/1961 Primary Examiner-Hyland Bizot Att0rney.lohn E; Miller 57] ABSTRACT Stainless steel with high resistance against corrosion and welding cracks, consisting of carbon, silicon, manganese, nickel, chromium, copper and the remainder of iron with an inevitable amount of impurity. The contents of the aforesaid ingredient elements of the stainless steel are so related with each other that the value of the following austenite/ferrite ratio is kept equal to or less than 1.06.

14 Claims, No Drawings STAINLESS STEEL WITH HIGH RESISTANCE AGAINST CORROSION AND WELDING CRACKS changer are kept in contact with pressurized corrosive fluid on one side surface thereof while exposing the op'- posite surface thereof to a fluid including halogen ions, such-as chloride ions. Under such conditions, the stainless steel sheets are exposed to the risk of corrosion throughout the entire surface thereof (to be referred to as general corrosion, hereinafter), as well as the risk of stress corrosion cracking,which may lead to failure of the equipment. i

As th'e'size of equipment becomes large in chemical industries, the use of automatic welding has been expanding in making such equipment. Due-to the need of quick delivery and efficient manufacture, automatic welding has increasingly been used. With the automatic welding, it is difficult to change welding conditions according to the weldability of the steel being used, as compared with manual welding. In addition, during automatic welding, curing'of the material is prone to negligence. As a result, stainless steel equipment in chemical industries are exposed to shortcomings of automatic welding process, such as welding bead cracks and stress corrosion cracks caused by residual stress of welding.

Thus, stainless steel ,material for chemical industry equipment is required to have threekinds of properties,

i.e., properties to resist against general corrosion, stress corrosion cracks, and welding bead cracks. There has not been any stainless steel hitherto which is provided composition 0.03 2.00 percent by weight of one or more elements selected from the group consisting of niobium, tantalum, and titanium.

It has generally been believed that-elements which tend to increase the risk of welding cracks of stainless steel (to be referred to as "weldability-reducing elements, hereinafter) are silicon, copper, phosphorus, and niobium. In order to reduce the risk of welding cracks, it has been a practice to minimize the contents of the weldability-reducingelements. If it is required to add in stainless steel a considerable amount of some of the weldability reducingelements for certain reasons,

.e.g., for taking advantage of special properties of the elements, the users of such stainless. steel have to sacrifree the resistance against welding cracks to some extent.

Apart from the effects on the weldability, the aforesaid weldability-reducing elements have the following specific properties.

A small amount of silicon is used in a steel mill as a deoxidizer. Silicon is also known as'a ferrite-forming element. The addition of a comparatively large amount of silicon improves the resistance of stainless steel against stress corrosion cracks in acidic chloride media such as magnesium chloride solution. v

The addition of 0.5 5 percent of copper in stainless steel improves its corrosion resistance, especially the general corrosion resistance in a non-oxidizing solution and stress corrosion cracking resistance in mildly corrosive chloride solution.

Niobium acts to fix nitrogen the structure of stainless steel, so as to improve the resistance against stress corrosioncracks. Niobium also combines with carbon' and thus reduces the grain boundary corrosion.

' As described above, the aforesaid elements have hitherto been considered detrimental to the weldability v of stainless steel, but it has not been found that these with all of the aforesaid three kinds of properties, al-

though conventional stainlesssteel materials alternatively have either one of the three kinds of properties.

The inventors have made a series, of studies on stainless steel for equipments and machines in chemical industries, such as welded stainless steel pipes for heat exchangers, and succeeded in developing improved stainless steel having theaforesaid'properties. The inventors confirmed by experiments that outstanding resistance against total surface corrosion, stress corrosion cracks, and welding bead cracks can be achieved by using stainless steel, which consists of, in percent by weight, 0.01 0.1 percent of carbon (C), 2 6 percent of silicon (Si), 0.01 3 percent of'manganese (Mn), 0.5 5 percent of copper (Cu), 7 20 percent of nickel (Ni), 13 25 percent chromium (Cr), and the remain-- der of iron with impurities, under conditions of The inventors also confirmedthat the corrosionresistance, especially resistance against stress corrosion, can further be improved byadding into the aforesaid elements can exist together in stainless steel to improve corrosion resistance thereof, without impairing weldability thereof.

The inventors have confirmed that silicon and copper can be-used for improving the corrosion resistance of stainless steel, while avoiding their adverse effects of deteriorating the resistanceof stainless steel against welding cracks. In order to'avoid the increase of welding cracks ,'.the amount of ferrite 'phase being formed during the welding process is controlled so as to be more than 1.2percent, so that welding cracks, especially high temperature bead cracks, can be prevented even under severe welding conditions. In other words, according to the present invention, some of the elements,which have been consideredito accelerate welding cracks are used for suppressing the welding cracks, based on the following findings of the inventors.

If a stable austenic stainless steel containing a large amount of both silicon and copper in its stable austenite single phase is-welded, both of these elements tend to be concentrated. in austenite grain boundary during the solidifying after thewelding. As can be seen from the equilibrium diagram of a ternary Fe-Cr-Ni system, the range of composition of austenite phase produced by cooling molten steel is very wide under certain conditions; namely when austenite is in the form of austenic single phase composition prepared at a high temperature, when austenite initially I contains a small amount of ferrite and the ferrite is diminished by peritectic reaction, or' when the austenite phase is formed in a similar process. Due to the wide range of austenite composition, when austenite is formed by cooling molten stainless steel, the composition of the initially crystallized austenite phase greatly differs from that of the finally solidified portion thereof. Accordingly, as can be expected from the ternary Fe-Cr-Ni equilibrium diagram, the finally solidified portion of the austenite phase tends to contain a comparatively large amount of nickel crystallized therein. Silicon and copper, which are easily soluble in nickel, remain in the molten steel together with nickel until they are segregated in the finally solidified portion of the austenite phase, i.e., austenite grain boundary. In other words, if a large amount of silicon and copper exist in a stainless steel composition, together with nickel which readily dissolves silicon and copper, the silicon and copper tend to be segregated upon welding, and the segregated elements cause welding bead cracks when the stainless steel is subjected to stress during welding.

Thus, in order to prevent the welding bead cracks, it

has been considered to be desirable to minimize contents of silicon, copper and the like in stainless steel, so as to restrain the metals readily soluble in nickel, such as silicon and copper, from segregating along the grain boundaries.

The inventors, however, noticed the fact that silicon and copper can be added in stainless steel composition without causing any increase in welding bead cracks, if the concentration of the silicon and copper in austenite grain boundaries of stainless steel can be kept at a-low level. The inventors have succeeded in greatly reducing the concentration of silicon and copper by taking advantage of the phase diagram of the Fe-Cr-Ni ternary systems.

According to a process using the peritectic reactions .of the -Fe-Cr-Ni ternary system, the primary crystal from a molten steel of the ternary system includes a comparatively large amount of proeutectic ferrite containing a comparatively small amount of nickel, and then peritectic reactions occur between the proeutectic ferrite and the liquid phase steel for producing a new austenite phase, whereby the grain size of crystallized grains becomes smaller, The range of the austenite phase composition is considerably small, as compared with that of the austenite single phase without any crystallization of ferrite phase. Thus, concentration of nickel along the grain boundary is not remarkable. As a result, the concentration of silicon and copper in the stainless steel becomes smaller.

For a better understanding of the invention, reference is made to the accompanying drawings, in which:

FIGS. 1A and 1B are graphs showing distributions of copper and silicon in a stainless steel structure with a comparatively small amount of ferrite and a stainless steel structure with a comparatively large amount of ferrite, respectively; and

FIG. 2 is a graph showing the relation between the amount of ferrite in welded structures of stainless steel and the austenite/ferrite ratio thereof.

The inventors carried out tungsten inert gas (TIG) welding by using argon gas on l8Cr/l2Ni stainless steel containing 3.04 percent of silicon and 1.47 percent of copper, as well as on 18Cu/12Ni stainless steel containing 3.46 percent of silicon and 1.42 percent of copper. The degree of segregation of silicon and copper in welded beads wasmeasured by using a microprobe analyser. Measurement was also made on the content of ferrite in the welded beads, so as to find the relation between the production of ferrite and the degree of segregation of silicon and copper. The results are shown in FIGS. 1A and 1B.

FIG. 1A shows the degree of segregation of silicon and copper in beads made by the test welding of 18Cu/l2Ni stainless steel sample containing 3.04 percent of silicon and 1.47 percent of copper. With this sample of stainless steel, the amount of residual ferrite phase was always less than 1.2 percent, and austenite grain boundaries with high concentrations of copper and silicon were formed in the welded beads, as shown in FIG. 1A. During the welding of this sample, beadcracks were formed at the austenite grain boundaries.

Curves of FIG. 1B represent theresults of welding tests on the 18Cr/ l 2Ni stainless steel sample containing 3.46 percent of silicon and 1.42 percent of copper, illustrating the degree of segregation of silicon and copper at welded beads of the sample steel. The amount of the ferrite phase in various parts of the beads proved to be 4 6 percent. In this steel sample, there was no localized concentration of silicon and copper in the welded beads, as shown in FIG. 18. There was no bead cracks caused by the welding of this steel sample.

It is apparent from the foregoing that segregation of silicon and copper, which is detrimental to successful welding, can be prevented by providing a small amount of ferrite phase in an austenite matrix of stainless steel structure at welding portions. The formation of the ferrite phase in the stainless steel structure being welded, however, mainly depends on the composition of the stainless steel being welded.

Welding tests were made to find'the relation of the amount of ferrite phase in the austenite matrix and the bead cracks caused by welding, by conducting TIG au- The formula 1 is derived by considering the fact that nickel, manganese, and carbon act as austenitestabilizing elements in the stainless steel, while chromium and silicon act as ferrite-stabilizing elements. The ordinate of FIG. 2 represents the content of ferrite phase in the welded stainless steel structure. Cross marks (x) in FIG. 2 represent those samples which produced welding bead cracks, while the circle marks (0) represent those samples which were free from welding bead cracks.

If a noticeable amount of molybdenum is contained in the stainless steel, the following formula (1) is used as the values to be represented on the abscissa of FIG. 2.

be represented on the abscissa of FIG. 2.

ination of manganese does not provide any substantial gain in the properties of stainless steel.

Copper, 0.5-'5 percent: Copper is effective in improving the resistance against the general corrosion. It is -w OEMMQB) +300(%) In the following description, the values of the formulae l, 1', and 2 will be referred to as the austenite/ferriteratio, because the numerical values of the formulae are indicative of the possible amount of ferrite to be produced in the stainless steel structure upon welding.

creases, the welded structure becomes closer to austenite single phase structure, so that the risk' of welding bead cracks increases, when a large amount of silicon,

7 copper, or niobium is present.

Accordingly, in order to prevent the welding bead cracks, the austenite/ferrite ratio should be restricted to be smaller than a certain value. According to the results of experiments carried out by the inventors, there was no welding bead cracks when the austenite/ferrite ratio was 1.06 or smaller, as shown in FIG. 2. Welding bead cracks were experienced when the austenite/ferrite ratio was larger than 1.06.

Thus, welding bead cracks can be prevented by controlling the composition of-stainless steel, while using the aforesaid austenite/ferrite ratio as a scale, for instance by limiting the austenite/ferrite ratio at a value not greater than 1.06.

According to the present invention, the composition of stainless steel is defined by further considering the corrosion resistance, shapability, and other properties thereof, as will be described hereinafter. In other words, according to the present invention, in order to prevent the welding bead cracks of stainless steel while ensuring high corrosion resistance thereof, the content of each of ingredient elements of the stainless steel should fall within the following range, provided that the aforesaid austenite/ferrite ratio is kept at 1.06 or smaller than 1.06.

Carbon, 0.01-0.1 percent: Stainless steel of the invention becomes very susceptible to stress corrosion cracking if the carbon content is less than 0.01 percent,

while more than 0.1 percent of carbon can cause intergranular corrosion during pickling of stainless steel. Silicon, 2-6 percent: The addition of silicon in stainless steel is necessary for formation of ferrite phase therein, which ferrite phase is effective in preventing welding bead cracks, and for improving the resistance against stress corrosion cracks. Less than 2 percent of silicon is not effective for improvement of stress corrosion resistance and formation of the desired amount of ferrite, while the addition of silicon in excess of 6 percent gives adverse effects on malleability. Manganese, 0.01-3 percent: It is a general practice in steel industry to add manganese in steel at a rate less than 3 percent. Manganese contained in starting materials for steel making is carried into steel products, and in eliminating such manganese, it is difficult to reduce its content to less than 0.01 percent, while further elimalso effective in preventing pitting and stress corrosion cracking in mildly corrosive chloride solution, such as sodium chloride solution. Less than 0.5 percent of copper does not result in any noticeable improvement of the corrosion resistance, while the addition of copper in excess of 5 percent tends to give adverse effects on the malleability and weldability of stainless steel by precipitating deposits mainlyconsisting of copper.

For improving the resistance against the general and pitting corrosion, molybdenum is also effective, but molybdenum has drawback in that it increases stress corrosion cracks. Accordingly, it is preferable to use copper for improving the resistance against the general and pitting corrosion while minimizing the molybdenum content therein.

The addition of copper in excess of 0.5 percent also improves the shapability of stainless steel in cold workmg.

Chromium, 13-25 percent; nickel, 7*20 percent: Chromium and nickel are essential ingredients of stainless steel for ensuring the corrosion resistance and mechanical strength thereof. Chromium content less than 13 percent results in considerable deterioration of the resistance against general andpitting corrosion, while the addition of chromium in excess of 25 percent tends to produce detrimental sigma phase in the stainless steel. the minimum content of nickel to ensure satisfactory resistance against stress corrosion cracks is 7 percent, while the addition of nickel in excess of 20 percent sharply increases the cost of stainless steel without producing any material improvement of the corrosion resistance.

Other elements:

Preferably, it is useful to add 0.03 2 percent of one or more elements from the group of niobium, tantalum, and titanium. The addition of such elements can eliminate intergranular corrosion and also increases stress corrosion cracking resistance by combining with nitrogen, which is detrimental to stress corrosion cracking if it is dissolved in solid solution. Accordingly, their addition is preferable. The effects of niobium, tantalum, and titanium are substantially equivalent to each other, and hence, any one or two or three can be used. Less than 0.03 percent of such element or elements is not effective in improving the resistance against the intergranular corrosion. Stainless steel usually contains less than 0.03 percent of such element or elements as impurities left from the starting materials. The addition of more than 2 percent of one or more of niobium, tantalum, and titanium produces a noticeable amount of compounds with other elements, which are detrimental to the manufacture of satisfactory stainless steel.

Phosphorus should be limited to less than 0.04 percent, because it tends to accelerate the stress corrosion cracks of stainless steel. Similarly, the content of molybdenum is preferably limited to less than 0.1 percent, due to its inclination to accelerate the stress corrosion, although molybdenum improves the resistance against general corrosion. If a certain degree of stress corrosion cracks should be allowed, up to 0.5 percent of molybdenum can be contained in the stainless steel, e.g.,

as a part of impurities carried from melting materials.

A very small amount of sulfur may be contained in stainless steel, as a part of impurities thereof. Since such small amount of sulfur does not affect the corrosion resistivity, sulfur content up to a level normally achievable by conventional steel making process is allowable.

The stainless steel according to the present invention will now be described in further detail, referring to examples.

Example 1 Stress corrosion cracks of the stainless steel of the present invention were compared with those of known stainless steel, by using samples, as listed in Table 1. For the purpose of simplicity of the comparison, some of Samples of known stainless steel were so selected that contents of individual ingredients lie within those according to the present invention but their austenite/ferrite ratios exceed 1.06, such as Samples H, I, and M. Some other samples of known stainless steel have compositions outside of the composition range of the present invention, together with austenite/ferrite ratios smaller than 1.06, such as Sample F. The remaining Samples of known stainless steel have compositions and austenite/ferrite ratios, both different from those of the present invention, such as Samples G, J, K, L, N, O, P, Q, R, and S.

1t is apparent from Table 2 that the stainless steel according to the present invention (Samples A to E) did not produce any stress corrosion cracks even after 300 hours of loading under the constant load conditions. Similarly, stainless steel of Samples F M, which contained 2 percent or more of silicon, did not produce any stress corrosion cracks in 300 hours of the aforesaid constant load test. On the other hand, those samples, whose silicon content was less than 2 percent, experienced stress corrosion cracks in less than 150 hours of the aforesaid constant load test.

In the constant strain test in sodium chloride solution, stainless steel Samples containing both 2 percent or more of silicon and 0.5 percent or more of copper did not experience any stress corrosion cracks at all, while Samples containing less than 2 percent of silicon (e.g., Samples N and O) or the Sample which contains only 0.01 percent copper (Sample F) experienced some stress corrosion cracks.

Example 2 Samples of Table l were prepared in the same manner as Example 1, and general and pitting corrosion tests were made on them. The results are shown in Table 2.

The corrosion tests were carried out in three different processes: namely, measuring the corrosion (gr/m hr) of each sample by dipping it in a boiling solution TABLE 1 Kind Composition e of Sample A/F steel Symbol C(%) Si(%) Mn(%) Ni(%) Cr(%) Mo(%) Cu(% l ratio Stainless steel A 0.04 3.51 0.61 12.65 18.60 1.44 0.89 of the invention B 0.05 3.57 0.68 12.65 18.88 0.003 1 47 0.88 C 0.04 3.02 0.55 13.90 19.87 1.44 0.93 D 0.05 3.19 0.62 12.60 18.68 0.03 1.55 0.92 E 0.05 3.42 0.51 13.06 18.35 0.70 0.94

Known F 0.05 3.47 0.50 13.06 18.35 0.01 0.93 stainless steel J 0.06 2.58 1.49 20.54 14.02 3.24 2.06 K 0.07 2.80 1.62 20.25 14.78 3.38 1.88 L 0.06 3.39 1.49 20.19 13.87 4.08 1.85

N 0.06 0.71 1.70 9.03 18.46 f O 0.07 0.66 1.85 12.31 16.83 2.65 P 0.06 0.58 1.71 12.83 17.67 2.31 2.08 Q 0.13 0.85 1.59 21.02 25.02 R 0.05 0.79 1.73 21.81 17.72 2.25 1.49 S 0.04- 0.58 1.55 22.11 24.42 2.05

' Sec equation (1).

The stress corrosion tests were conducted under constant strain and under constant load. In the case of constant load tests, samples of Table l were dipped in a solution of concentrated solution of magnesium chloride, which was kept at 150C, and tension stress was applied to Samples in the solution. The tension varied in a range of 25 kg/mm 1n the case of constant strain tests, Samples were dipped in a boiling salt water for 500 hours, which salt water contained 20 percent of sodium chloride and 1.5 percent of sodium bichromate. The resultsare shown in Table 2.

TABLE 2 Kind of Sample Constant pressure, Breaking time Constant strain,

Corrosion in Total surface corrosion test by dipping Welding test Corrosion in Corrosion in Cracks Cracks tained only a very small amount of copper (Samples F,

G) or no copper at all (Samples N, O), experienced a considerably large extent of general corrosion. The resistance of stainless steel against sulfuric acid varied depending on the content of copper therein.

The tests with the 10 percent hydrochloric acid at 30C proved that the stainless steel samples'of the present invention had a high resistance against hydrochloric acid, while Samples which contained a small amount'of copper or no copper (Samples G, N, experienced considerable general corrosion.

According to the results of tests with percent ferric chloride solutions at 40C, which were intended to check the pitting corrosion resistance, samples of the 1 present invention proved tohavean improved resistance against such salt.

The critical current density for passivation was measured on Sample A, representative of the stainless steel of the present invention, and on Sample F, representative of conventional stainless steel, in a deaerated 5 percent sulfuric acid solution at C by potentiostatic method. The critical current density for passivation of Example 3 0 Samples A-M, as shown in Table 1,. were shaped into steel Symbol in MgCl, (hours) cracks in boiling 5% 10% 10% ferric on on samples in salt sulfuric acid hydrochloric chloride solution welding welding water acid at 30C at 40C face- 7 backbeads beads Stress 25 Stress 35 (gr/m hr) (gr/mhr) (gr/mhr) Kgl 2 Kngfimmz Stain- More than 300 More than 300 None 24.0 0.90 3.49, 5.36 None None less steel of the invention B 24.0 0.87 2.48, 5.58 C 18.0 1.03 4.07, 4.54 D 16.9 5.03, 5.09 E 17.5 2.01, 2.03

' Known F More than 300 More than 300 Some stainless steel (3 2.36 14.2 A few A few H More than 300 More than 300 None A few A few J More than 300 More than 300 None 797, 980 Many Many K 5.35

N l, l Some 37.9, 39 5 3.06 26.8 O 6, 6 5 37.4, 37 7 4.14, 6.63 l4 '.7 P 6, 7 None 0 119,154 38,40 Some R 36, 37 23, 25 S 20, 24 15, 26'

3 inm thick sheets, and the sheets thus shaped were subjected to tungsten inert gas'(TlG) welding. The welding was carried out by an automatic welder using tungsten electrodes and argon gas. The cracks on welding beads formed by the T10 welding were examined by color check. The results are shown in Table 2.

The welding conditions include a welding current of A, a welding speed of 200 mm/min., and an argon ejection at 12 lit./min.

According to the results of the welding test, all samples with an austenite/ferrite' ratio, which is smaller than 1.06 (Samples A-F), did not experience any welding cracks both at face-beads and back-beads.

Even when the austenite/ferrite ratio exceeds 1.06, if its difference from 1.06 is small (e.g., Samples G, H, l), the number of welding bead cracks is very small, although the welding bead cracks cannot be prevented completely. As the value of the austenite/ferrite ratio departs further from 1106, the number of welding bead cracks increases.

The rnechanicalstrength of the stainless steel of the invention was compared with that of a known stainless steel (Sample F) containing a small amount of copper. The results are shown in Table 3. It is apparent from the table that the stainless steel-according to the present invention has excellent mechanical strength. It was found that Sample F, which contained only 0.01 percent of copper, had a hardness higher than that of the stainless steel of the present invention.

TABLE 3 ltem Samples of the present invention Stainless steel of known composition Sample Sample A Sample B Sample C Sample D Sample F. Sample F symbol Shape Round bar Plate Round bar Plate Round bar Plate Plate Plate Plate Yield (0.2% 24.1 28.6 28.5 29.5 23.5 27.6 29.2 28.8 28.0 offset) Tensile 6l.9 66.6 64.8 67.5 57.7 61.6 65.8 65.9 67.1

strength Elongation 69.6 61.6 69.2 58.8 69.0 59.8 59.0 58.6 62.0

Contraction of 73.6 75.7 75.0

area

Hardness HB HV HB HV HB HV HV HV HV 152 165 147 164 139 157 161 174 186 1-18 represents a Brinell hardness number, while HV represents a Vickers hardness number.

What is claimed is:

l. A stainless steel heat exchanger for receiving corrosive chemicals, a portion of the body of said heat exchanger adapted to come into contact with said corrosive chemicals, said portion having a welded joint therein made by continuous inert gas arc welding with non-consumable tungsten electrodes, the stainless steel of said portion consisting essentially of 0.01 to 0.1 percent by weight carbon, 2 to 6 percent by weight silicon, 0.01 to 3 percent by weight manganese, 7 to percent by weight nickel, 13 to percent by weight chromium, 0.5 to 5 percent copper and the remainder iron,

4. A heat exchanger according to claim 1 wherein the stainless steel of said portion consists essentially of in percent by weight, 0.01 to 0.1 percent carbon (C), 2 to 6 percent silicon (Si), 0.01 to 3 percent manganese (Mn), 7 to 20 percent nickel (Ni), 13 to 25 percent chromium (Cr), 0.5 to 5 percent copper (Cu), up to 0.1 percent molybdenum (Mo), 0.03 to 2.00 percent of at least one element selected from the group consisting of niobium (Nb), tantalum (Ta), and titanium (Ti), and the remainder of iron with an inevitable amount of impurity, said contents of the elements in the stainless steel satisfying the relationship the contents of the elements in the stainless steel satisfy 5, A a ti l a f t h i a w ld d j i t the relationship.

2. A heat exchanger according'to claim 1 wherein the stainless steel of said portion consists essentially of 0.01 to 0.1 percent by weight carbon (C), 2 to 6 percent by weight silicon (Si), 0.01 to 3 percent by weight manga- 3. A heat exchanger according to claim 1 wherein the stainless steel of said portion consists essentially of in percent by weight, 0.01 to 0.1 percent carbon (C), 2 to 6 percent silicon (Si), 0.01 to 3 percent manganese (Mn), 7 to 20 percent nickel (Ni), 13 to 25 percent chromium (Cr), 0.5 to 5 percent copper (Cu), 0.03 to 2.00 percent of at least one element-selected from the group consisting of niobium (Nb), tantalum (Ta), and titanium (Ti), and the remainder of iron with an inevitable amount of impurity, the contents of the elements in the stainless steel satisfying the relationship therein joining two edges of material together, said welding joint produced by subjecting said two edges of continuous inert gas arc welding with nonconsumable tungsten electrodes, said edges composed of a stainless steel each independently consisting essentially of by weight, 0.01 to 0.1 percent carbon, 2 to 6 percent silicon, 0.01 to 3 percent manganese, 7 to 20 percent nickel, 13 to 25 percent chromium, 0.5 to 5 percent copper and the remainder iron, the contents of the elements in the stainless steel satisfying the relationship Ni(%)+0.5Mn(%)-l-3OC(%)+2 Cr(%)+1.5Si(%)-5.6

6. The article of manufacture of claim 5 wherein said stainless steel consists essentially of by weight 0.01 to 0.1 percent by weight carbon (C), 2 to 6 percent by weight silicon (Si), 0.01 to 3 percent by weight manganese (Mn), 7 to 20 percent by weight nickel (Ni), 13 to 25 percent by weight chromium (Cr), 0.5 to 5 percent by weight copper (Cu), up to 0.1 percent by weight molybdenum (Mo), and the remainder iron with an inevitable amount of impurity, the contents of the elements in the stainless steel satisfying the relationship 7. The article of manufacture of claim 5 wherein said stainless steel consists essentially in percent by weight 0.01 to 0.1 Percent Carbon 2 t 6 Percent Silicon 10. A process according to claim 9 wherein said inert (Si), 0.01 to 3 percent manganese (Mn), 7 to 20 pergas i argon C n nickel 13 to 25 Percent chromium 11. Stainless steel having high resistance against to 5 Percent pp to P of at stress corrosion cracks and against welding cracks, eslea 0.116 element Selected fl'om the group consisting of 5 sentially consisting of, in percent by weight, 0.01 to 0.1 niobium tantalum and titanium (Ti), and percent of carbon 2 to 6 percent of silicon (Si), 0.01 to the remainder of iron with an inevitable amount of im- 3 percent of manganese (Mn) 7 to 20 percent ofnickel purity, the contents of the elements in the stainless steel (Ni), 13 to 25 percent of chrmium y 05 to 5 Pen Satisfying the relationship cent copper (Cu), and the remainder of iron with an 8. The article of manufacture of claim 5 wherein said inevitable amount of impurity, said contents of the elestainless steel consists essentially in percent by weight 15' ments in the stainless steel satisfying the relationof 0.01 to 0.1 percent carbon (C), 2 to 6 percent of silicon (Si); 0.01 to 3 percent manganese (Mn), 7 to percent nickel (Ni), 13 to percent chromium (Cr), 0.5 to 5 percent copper (Cu), up to 0.1 percent molyb- +0.5Mn(%) +C(%) +2 denum (Mo), 0.03 to 2.00 percent of at least one ele- 20 Z1)'.i'1.5Si(%)5.6 =106 ment selected from the group consisting of niobium (Nb), tantalum (Ta), and titanium (Ti), and the remainder of iron with an inevitable amount of impurity, 12. The stainless steel of claim 11 further comprising said contents of the elements in the stainless steel satisup to 0.1% molybdenum, the contents of the elements fying the relationship 25 in said stainless steel satisfying the relationship +0.5Mn(%) =1 06 Cr(%)+1.5Si(%)+Mo(%)+0.5Nb(%)+0.5Ti(%)+0.5Ta(%)-5.6

9. A process for forming a welded article having high 30 resistance against (a) general corrosion, (b) stress corrosion cracks and (c) welding bead cracks comprising causing two edges of one or more pieces of stainless steel to come together in welding relationship and welding the edges together by continuous inert gas a 5 13. The stainless steelof claim 11 further comprising welding with nonconsumable tungsten electrodes, id 0.03 to 2.00% of at least one element selected from the process characterized in that said stainless steel congroup Consisting of niobium tantalum and sists essentially of 0.01 to 0.1 percent by weight carbon, titaflillm the Contents of'the elements in Said Slain- 2 to 6 percent by weight silicon, 0.01 to 3percent by l steel satisfying the relationship weight manganese, 7 to 20 percent by weight nickel, l3 14. The stainless steel of claim 13 further comprising to 25 percent by weight chromium, 0.5 to 5 percent up to 0.1% molybdenum (M0), the contents of the elecopper and the remainder iron, the contents of the elements in said stainless steel satisfying the relationship ments in the stainless steel satisfying the relationship Cr(%)+1.5Si(%)-5.6 i *f 

2. A heat exchanger according to claim 1 wherein the stainless steel of said portion consists essentially of 0.01 to 0.1 percent by weight carbon (C), 2 to 6 percent by weight silicon (Si), 0.01 to 3 percent by weight manganese (Mn), 7 to 20 percent by weight nickel (Ni), 13 to 25 percent by weight chromium (Cr), 0.5 to 5 percent by weight copper (Cu), up to 0.1 percent by weight molybdenum (Mo), and the remainder iron with an inevitable amount of impurity, the contents of the elements in the stainless steel satisfying the relationship
 3. A heat exchanger according to claim 1 wherein the stainless steel of said portion consists essentially of in percent by weight, 0.01 to 0.1 percent carbon (C), 2 to 6 percent silicon (Si), 0.01 to 3 percent manganese (Mn), 7 to 20 percent nickel (Ni), 13 to 25 percent chromium (Cr), 0.5 to 5 percent copper (Cu), 0.03 to 2.00 percent of at least one element selected from the group consisting of niobium (Nb), tantalum (Ta), and titanium (Ti), and the remainder of iron with an inevitable amount of impurity, the contents of the elements in the stainless steel satisfying the relationship
 4. A heat exchanger according to claim 1 wherein the stainless steel of said portion consists essentiaLly of in percent by weight, 0.01 to 0.1 percent carbon (C), 2 to 6 percent silicon (Si), 0.01 to 3 percent manganese (Mn), 7 to 20 percent nickel (Ni), 13 to 25 percent chromium (Cr), 0.5 to 5 percent copper (Cu), up to 0.1 percent molybdenum (Mo), 0.03 to 2.00 percent of at least one element selected from the group consisting of niobium (Nb), tantalum (Ta), and titanium (Ti), and the remainder of iron with an inevitable amount of impurity, said contents of the elements in the stainless steel satisfying the relationship
 5. An article manufacture having a welded joint therein joining two edges of material together, said welding joint produced by subjecting said two edges of continuous inert gas arc welding with nonconsumable tungsten electrodes, said edges composed of a stainless steel each independently consisting essentially of by weight, 0.01 to 0.1 percent carbon, 2 to 6 percent silicon, 0.01 to 3 percent manganese, 7 to 20 percent nickel, 13 to 25 percent chromium, 0.5 to 5 percent copper and the remainder iron, the contents of the elements in the stainless steel satisfying the relationship
 6. The article of manufacture of claim 5 wherein said stainless steel consists essentially of by weight 0.01 to 0.1 percent by weight carbon (C), 2 to 6 percent by weight silicon (Si), 0.01 to 3 percent by weight manganese (Mn), 7 to 20 percent by weight nickel (Ni), 13 to 25 percent by weight chromium (Cr), 0.5 to 5 percent by weight copper (Cu), up to 0.1 percent by weight molybdenum (Mo), and the remainder iron with an inevitable amount of impurity, the contents of the elements in the stainless steel satisfying the relationship
 7. The article of manufacture of claim 5 wherein said stainless steel consists essentially in percent by weight 0.01 to 0.1 percent carbon (C), 2 to 6 percent silicon (Si), 0.01 to 3 percent manganese (Mn), 7 to 20 percent nickel (Ni), 13 to 25 percent chromium (Cr), 0.5 to 5 percent copper (Cu), 0.03 to 2.00 percent of at least one element selected from the group consisting of niobium (Nb), tantalum (Ta), and titanium (Ti), and the remainder of iron with an inevitable amount of impurity, the contents of the elements in the stainless steel satisfying the relationship
 8. The article of manufacture of claim 5 wherein said stainless steel consists essentially in percent by weight 0.01 to 0.1 percent carbon (C), 2 to 6 percent of silicon (Si), 0.01 to 3 percent manganese (Mn), 7 to 20 percent nickel (Ni), 13 to 25 percent chromium (Cr), 0.5 to 5 percent copper (Cu), up to 0.1 percent molybdenum (Mo), 0.03 to 2.00 percent of at least one element selected from the group consisting of niobium (Nb), tantalum (Ta), and titanium (Ti), and the remainder of iron with an inevitable amount of impurity, said contents of the elements in the stainless steel satisfying the relationship
 9. A process for forming a welded article having high resistance against (a) general corrosion, (b) stress corrosion cracks and (c) welding bead cracks comprising causing two edges of one or more pieces of stainless steel to come together in welding relationship and welding the edges together by continuous inert gas arc welding with nonconsumable tungsten electrodes, said process characterized in that said stainless steel consists essentially of 0.01 to 0.1 percent by weight carbon, 2 to 6 percent by weight silicon, 0.01 to 3 percent by weight manganese, 7 to 20 percent by weight nickel, 13 to 25 percent bY weight chromium, 0.5 to 5 percent copper and the remainder iron, the contents of the elements in the stainless steel satisfying the relationship
 10. A process according to claim 9 wherein said inert gas is argon.
 11. Stainless steel having high resistance against stress corrosion cracks and against welding cracks, essentially consisting of, in percent by weight, 0.01 to 0.1 percent of carbon 2 to 6 percent of silicon (Si), 0.01 to 3 percent of manganese (Mn), 7 to 20 percent of nickel (Ni), 13 to 25 percent of chromium (Cr), 0.5 to 5 percent copper (Cu), and the remainder of iron with an inevitable amount of impurity, said contents of the elements in the stainless steel satisfying the relation of
 12. The stainless steel of claim 11 further comprising up to 0.1% molybdenum, the contents of the elements in said stainless steel satisfying the relationship
 13. The stainless steel of claim 11 further comprising 0.03 to 2.00% of at least one element selected from the group consisting of niobium (Nb), tantalum (Ta), and titanium (Ti), the contents of the elements in said stainless steel satisfying the relationship
 14. The stainless steel of claim 13 further comprising up to 0.1% molybdenum (Mo), the contents of the elements in said stainless steel satisfying the relationship 